1. Field of the Invention
The present invention relates to a method of manufacturing an automobile steering rack by rolling a side surface of a hollow pipe-like rack material with rack teeth.
2. Description of the Prior Art
A usual rack used for an automotive steering mechanism or the like is manufactured from a round bar by flattening a portion of the outer periphery of the bar and then forming rack teeth on the flattened portion. As the preferred rack material, a hollow pipe is used in order to satisfy a light weight demand.
Japanese Patent Publication (JP-B2) No. Sho 58-31257/1983 shows a method of forming rack teeth by plastically processing a rack material. In this method, a rack teeth formation portion of the material is pressed in a direction perpendicular to the axis of the material with an upper die having a tooth form which is complementary to the rack teeth.
Japanese Patent Publication (JP-B2) No. Hei 3-5892/1991 shows a method of forming a train of teeth on a flat portion of a hollow pipe-like rack material, by applying a forming die which has the same concavity and convexity as the rack teeth on the flat portion and then pressure fitting a punch into the hollow on the pipe. The flat portion of the material is forced into the concavity and convexity of the forming die.
However, both of these prior art techniques involve the preliminary step of forming a side portion of the hollow pipe with a flat surface. In addition, the forming process requires a high processing force (i.e., a high pressing force), wherein burrs are readily formed around the newly formed rack teeth.
Accordingly, the applicant has earlier proposed in Japanese Patent Application No. Hei 5-299091/1993 a method of rack manufacture that forms rack teeth with a comparatively low force, and in which less burrs are generated. In this technique, when manufacturing a rack with rack teeth formed in a side surface, a hollow pipe-like rack material is held with a core bar inserted in it, a forming roll with a rack teeth form forming on the outer periphery is held for rolling over the rack material, and mutual moving of the rack material and the forming roll is caused in the tangential direction of the forming roll, thus forming the rack material with the rack teeth.
In other words, in the above prior art technique (Japanese Patent Application No. Hei 5-299091/1993), when processing the hollow pipe-like rack material, a core bar is inserted in the pipe-like rack material in order to suppress unwanted deformation of the outer periphery of the material and to effectively flatten a portion of the outer periphery of the material in order to form the rack teeth on the flattened portion.
When the core bar is inserted in the rack material, as shown in FIGS. 7A and 7B, a sectional profile will be formed which follows the sectional profile the rack teeth formation portion of the product rack. The sectional profile has a material support surface which is spaced from the rack teeth formation portion of the rack material, the material support surface being an entirely flat surface. Referring to FIGS. 7A and 7B a problem solved by the invention will be described, where designated at 1 is the rack material, at 1A the rack teeth, at 2 the core bar, and at 2A the material support surface.
FIGS. 7A and 7B show a process of forming a hollow pipe-like rack material with rack teeth of rack by making use of the shape of core bar in the prior art technique noted above, FIGS. 7A and 7B showing an initial stage and a final stage, respectively, of the process.
The side surface of the rack material that is pushed by the addendum of the forming roll, is partly deviated toward the clearance with the core bar and also partly deviated toward the outer periphery of the material, and the remainder being deviated the bottom of teeth form of the forming roll.
In the initial stage of the process, deviation (a) towards the clearance, readily proceeds, and eventually the deformation is suppressed by the material support surface 2A of the core bar (FIG. 7A). In this stage, the deformation is retarded at each end of the core bar. Consequently, a clearance 3 is produced between the material and core bar, and deformation toward the bottom of the teeth form of the forming roll (rack teeth 1A) is insufficient. Further, a depression 4 is formed in the center of the addendum of the rack teeth.
By increasing the forming cycles for raising the rack teeth to a predetermined height, the deviation proceeds in the direction of reducing the clearance 3. Since at this time the material support surface of the core bar is an entirely flat surface, the distance between the point of action of force on the addendum of the teeth form of the forming roll and each core bar end is small, and considerable forming cycles are required until the final shape (FIG. 7B) is obtained. At the same time, deviation (a) toward the outer periphery of the material is increased, and a great force acts against the clamp force on the material outer periphery. The deviation toward the outer periphery of the material is cut off when polishing the outer periphery of the material to a true circle after the rack forming process, thus resulting in waste of material.